In order to carry out an efficient conversion of a spectrum of electromagnetic radiation into electrical current, attempts have been made to develop commercial devices called monolithic cascade converters which provide multiple, typically two, semiconductor junctions within one device, one junction having a low characteristic electromagnetic energy gap and the other a high characteristic electromagnetic energy gap. Each of these junctions will be called sub-cells. The energies of the low and high energy gaps are chosen so as to optimize coverage of the incident electromagnetic spectrum, thereby maximizing the efficiency of converting incident electromagnetic radiation into electrical energy. For example, the theoretical maximum efficiency for a two-junction GaAs based device has been calculated to be in excess of 35% near ambient temperature for a typical solar spectrum measured at the earth's surface.
A number of complex connection methods have been proposed for connecting the sub-cells together. One such proposed method is called the metal-interconnected cascade cell, or MICC. See, for example, the paper of Ludowise et al, "High-Efficiency Organometallic Vapor Phase Epitaxy AlGaAs/GaAs Monolithic Cascade Solar Cell Using Metal Interconnects," Appl. Phys. Lett. 41(6) 550-552, Sept. 15, 1982. Metallization schemes such as the MICC involve complex and tedious processing (photolithography, etching, metallization), generally leading to low yields (about 10%) of successfully processed cascades. The extension of this MICC procedure to connect more than two sub-cells in series would involve extremely complex low yield processing and would significantly increase the metal grid shadowing of active cell areas.
Another proposed method of connection uses a germanium interconnect layer grown between the sub-cells. The low band gap of the Ge is said to permit the facile formation of tunnel junctions in the Ge which would possess the necessary high ohmic conductance. See for example the paper of Fraas, "A Shorting Junction for Monolithic Multicolor Solar Cells," Proc. 15th IEEE Photovoltaic Specialists Conference, Orlando, Fla., May 1981, pp. 1353-1356. The germanium tunnel junction is vulnerable to degradation via dopant diffusion during subsequent high-temperature processing steps. This is illustrated by some calculations by Fraas (Proc. 13th IEEE Photovoltaic Specialists Conference, 1978, Washington, DC, pp.
886-891). At a p-n tunnel junction, carriers tunnel through a barrier of width W. The junction resistance is extremely sensitive to changes in W; e.g., increasing W by only 20 .ANG. increases the resistance by an order of magnitude. For example, a tunnel junction of depletion width 125 .ANG. and conductivity 10 A/V-cm.sup.2 (sufficient to support a cascade operating at approximately 100 suns concentration, with less than 3% total power loss due to the tunnel junction) degrades to a conductivity of 1 A/V-cm.sup.2 at 145 .ANG. thickness, and 0.1 A/V-cm.sup.2 at 165 .ANG. thickness. The 165 .ANG. junction will support the cascade only under 1 sun and is too resistive for use under concentration. Further, junction smearing (e.g., by dopant diffusion) will render the junction useless as an interconnect. Unfortunately, this degree of diffusion can occur extremely readily under cascade growth and processing conditions.
The use of highly doped, abrupt junctions (tunnel diodes or backward diodes) to serve as high conductance interconnects has also been described. See for example the paper of Miller et al, "GaAs/AlGaAs Tunnel Junctions for Multigap Cascade Solar Cells," J. Appl. Phys. 53(1) 744-748, Jan. 1982; Bedair et al, "AlGaAs/GaAs High Efficiency Cascade Solar Cells," Proc. 15th IEEE Photovoltaic Specialists Conference, Orlando, Fla., May 1981, pp 21-26. Such highly doped, abrupt junctions have not yet been shown to be stable under cascade growth conditions. The successful use of such junctions requires that the high-conductance III-V characteristic (signifying degenerate doping levels on either side of the junction and an exceedingly abrupt doping profile) be retained during growth of the full cascade. If the Eg of the interconnect material is lower than that of the overlying sub-cell, the interconnect total thickness must be extremely thin (.ltoreq.125 Angstroms n-type or .ltoreq.150 Angstroms p-type) in order that it absorb .ltoreq.3% of the radiation destined for the underlying sub-cell(s). The degradation of high conductance behavior, caused by dopant diffusing during the growth conditions required for the full cascade, has been virtually impossible to avoid.
The use of superlattices to short junctions via defect tunneling, and thus create a high conductance junction, has been described in theory, but to my knowledge has never actually been constructed. See, for example, U.S. Pat. No. 4,278,474 to Blakeslee, et al. To date no working prototype device exists using a superlattice to actually short out a junction.
Another recent type of cell employs a shorted junction which relies on defect states, but does not use a true superlattice, to short the unwanted junction between a lower Si cell and an upper GaAs cell. The shorting junction functions because of defect tunneling. The defects are introduced via the lattice mismatch between the Si of the lower cell and Ge in the shorting layer. No metal is used in the junction. (See B.Y. Tsaur, et al, Proceedings 17th IEEE Photovoltaic Specialists Conference, Orlando, Fla., May 1-4, 1984, pp 440-444).
The performance of a solar cell is usually described in terms of fill factor, V.sub.oc, and I.sub.sc defined as follows. V.sub.oc is the voltage at zero current or open circuit voltage. I.sub.sc is the current at zero voltage or at short circuit. The fill factor is a measure of the "squareness" of the power characteristics, and of what fraction of V.sub.oc and I.sub.sc are contributing to cell power output. If I.sub.m and V.sub.m are the current and voltage at maximum power output, then fill factor is defined as the product of I.sub.sc times V.sub.oc divided by the product of I.sub.m times V.sub.m.